Generally, microphone devices used for carrying out collection of sound radiated from the sound source are required to have various directivities in dependency upon use purposes.
In view of the above, as the conventional microphone devices, there are a non-directional microphone device having fixed sensitivity also with respect to sound sources located in all directions, and a directional microphone device having fixed sensitivity with respect to a sound source located in a fixed direction. As the directional microphone device, a uni-directional microphone device in which sensitivity in the main axis direction of the directivity of the microphone unit is caused to be 1 and sensitivity in a direction perpendicular to the main axis is caused to be 0.5, and a ultra directional microphone device in which sensitivity in a direction perpendicular to the main axis of the microphone unit is caused to be 0.5 or less are provided.
As the microphone device having ultra directional property (directivity), there are known so called a shot-gun type microphone device which is the uni-directional type microphone device in which the microphone unit is attached to one end of a cylindrical sound (acoustic) tube having through holes or slits for sound collection bored at the peripheral surface thereof with the main axis of direction of the directivity being directed to the axis direction of the sound tube, and a microphone device of the secondary sound pressure gradient type in which two uni-directional microphone units are linearly arranged in the main axis direction.
The above-described so called shot-gun type microphone device presents (exhibits) ultra directional property (directivity) mainly in the high-pitched tone (sound) region (range), whereas the secondary sound pressure gradient type microphone device presents the ultra directional property (directivity) mainly in the low-pitched tone (sound) region (range).
Meanwhile, since the microphone devices used for voice (speech) input means of the speech (voice) recognition equipment used in the car navigation system or computer, etc. in which inputting of various command signals is carried out by utilizing voice are used with a view to exclusively collecting only voice of the human being subject to speech recognition, sharp directivity is required in the middle-pitched tone region (range).
Further, the uni-directional microphone devices conventionally used are such that noises from the side surface (lateral) direction except for the direction to which the main axis of the directivity direction of the microphone unit is directed are apt to be mixed. Therefore, such microphone devices are not suitable for use in the voice input means of the speech recognition equipment. Namely, since sounds except for sounds to be primarily collected would be collected, precise command signals could not be obtained.
In addition, the shot-gun type microphone device and/or the secondary sound pressure gradient type microphone device have the problems that not only it is impossible to satisfy sharp directivity in the middle pitched tone (sound) region (range) required for the voice input means of the speech recognition equipment, but also such devices are expensive.